Insulating Properties of Indiarubber. 503 



No. 1 consisted of cut sheet *068 cm. thick. This was 

 darker in appearance and rather less transparent than Para. 

 It cut rather more easily than most pure rubbers and was 

 less tenacious. On keeping it has become quite hard and 

 almost brittle. 



No. 2 was pure Para rubber *051 cm. thick. It was very 

 transparent and very tenacious, and has retained these pro- 

 perties even after keeping for two years. 



No. 3 was a vulcanized rubber, white on the surface and 

 dark when cut. It was *093 centimetre thick. 



The cable tested was a 3/18 in., 2760 feet long, insulated to 

 '208 in. diameter with vulcanized rubber, and having an outer 

 covering of impregnated braid. The sheet indiarubber was 

 made into condensers by fixing tinfoil on each side by means 

 of indiarubber solution made with pure benzene. The tinfoil 

 sheets were 8 in. X 6 in., the dielectric projecting along each 

 edge an inch beyond the tinfoil in order to prevent leakage. 

 Each condenser thus made was placed between two glass 

 plates, a strip of tinfoil being brought out from each sheet to 

 the upper side of the top glass plate in order to make con- 

 nexion. The surface of the glass was coated with shellac to 

 prevent leakage by condensation of moisture on the glass. 

 The paraffined paper condenser was made of stout notepaper 

 which was first dried in a hot oven and then dipped into 

 melted paraffin-wax. Four sheets of this paper 10 in. X 8 in. 

 were alternated with five sheets of tinfoil, and placed between 

 two glass plates. The condenser was then put between two 

 iron blocks heated to about 85° C, and thus the wax was 

 heated and allowed to solidify and cool down under pressure. 

 The mica used was very pure and transparent, and consisted 

 of sheets 2 J in. x 4 J in. An endeavour was made to form 

 the plates of the condenser by depositing a film of silver on 

 the mica from a solution of silver nitrate and Rochelle salt. 

 By this means, intimate contact would be made between the 

 metal and the mica without the introduction of any other 

 dielectric in the form of a fixing material, or the inclusion of 

 films of air. This, however, was not successful, as out of 

 30 sheets silvered not one was obtained which would give 

 steady readings when charged with a 100-volt battery. The 

 plates of the condenser were therefore formed of sheets of 

 tinfoil H in. x 3g in., which were alternated with the mica 

 sheets, and the tinfoil was kept in close contact with the 

 mica by pressure alone. This was effected by holding 

 the condenser between two brass plates v hich were bolted 

 together by 5/1(5 in. bolts, the condenser being insulated from 

 the frame by sheet ebonite. The mica condenser eventually 



2 L 2 



